The demand for ever-increasing data rates in communication systems dictates the use of large bandwidths. Orthogonal frequency-division multiplexing (OFDM) (see refs. [1], [2]) has become a widely accepted technique to significantly reduce receiver complexity in broadband wireless systems. Multiple standards employing OFDM modulation for the physical layer have emerged by now, among them the 802.11a/g wireless local area network (WLAN) standard (see ref. [3]), the 802.16 broadband wireless access (BWA) standard (see ref. [4]), and the European digital audio broadcasting (DAB) and digital video broadcasting standards (DVB-T). OFDM is also widely used in wireline systems, such as covered by the various xDSL standards. In wireless applications, OFDM is often combined with bit loading and/or precoding at the transmitter, and usually referred to as discrete multi-tone (DMT) modulation.
For wireless systems, multiple-input multiple-output (MIMO) systems employ multiple antennas at both the transmitting and receiving side of the radio link to improve spectral efficiency. Other MIMO systems can, e.g., transmit data in parallel through a number of cables with crosstalk between the same. MIMO techniques in conjunction with OFDM modulation (MIMO-OFDM) are popular for modern broadband communication systems and can be used to transmit at high data rates (see ref. [5]). MIMO-OFDM is, among others, under consideration in the 802.11n high throughput working group, which aims to extend the data rates of 802.11a/g based WLANs beyond the current maximum of 54 Mbit/s. As a wireline example for MIMO-OFDM systems, high-rate xDSL links are often impaired by crosstalk between wires running in parallel and can thus also be modeled as MIMO-OFDM systems. MIMO-OFDM transceivers are computationally very complex. For the implementation of practicable systems, it is, therefore, crucial to devise efficient algorithms for MIMO-OFDM.
Systems of this type, as shown in FIG. 4, generally have a transmitter and a receiver. The transmitter comprises MT transmitter units TU0 . . . TUMT−1 and the receiver comprises MR receiver units RU0 . . . RUMR−1.
A sequence of original data symbols is fed to the transmitter, each original data symbol comprising N>1 vectors ck with k=0 . . . N−1, wherein each vector ck has MT complex-valued coordinates (c0)k . . . (cMT−1)k to be fed to the transmitter units TU0 . . . TUdi T−1.
The original data symbols are OFDM-modulated in the transmitter units, the result of which is transmitted over a dispersive channel to the receiver units, which demodulate the channel output into received symbols. Each received symbol comprises N vectors rk with k=0 . . . N−1, wherein each vector rk has MR complex-valued coordinates (r0)k . . . (rMR−1)k. For a linear channel (or an approximately linear channel) we have,rk=√{square root over (Ex)}·H(sk)ck+wk,  (i)with
√{square root over (Es)} being a constant, namely the square root of an energy scaling factor Es,
sk=exp(−j2πk/N), j being the imaginary unit,
                                          H            ⁡                          (                              s                k                            )                                =                                    ∑                              l                =                0                                            L                -                1                                      ⁢                                          H                l                            ⁢                              s                k                l                                                    ,                            (        ii        )            
L an integer smaller than N,
Hl being an MR×MT matrix having coefficients (Hij)l describing the value at discrete-time delay l of the discrete-time impulse response of the channel between transmitter unit TUj and receiver unit RUi, and
wk being the noise vector of the system, which is assumed to be a complex valued circularly symmetric white Gaussian zero-mean noise.